1. Field of the Invention
The present invention relates generally to asphalt drum mixers. More particularly, the present invention relates to an asphalt drum mixer having a self-cleaning drum.
2. Background of the Related Art
Asphalt drum mixers having a counterflow dryer and a separate mixing device are required in some jurisdictions to meet strict air pollution laws. In these highly desirable asphalt drum mixers, the drum is constructed so that the heated gas flows counter to the direction of movement of the aggregate. A mixer of this type is disclosed in U.S. Pat. No. 4,867,572 to Brock et al. More particularly, in the drum mixer of the Brock et al. patent, a fixed sleeve surrounds the lower portion of the rotating drum so that the heated and dried aggregate is discharged into the annular chamber which is formed between the drum and sleeve. Also, an inlet is provided in the sleeve by which recycled asphalt pavement (RAP) may be introduced into the annular chamber, and another inlet is provided to introduce liquid asphalt into the annular chamber. The drum mounts mixing blades which are positioned in the annular chamber to mix the materials and cause them to be moved longitudinally to the discharge outlet of the sleeve.
The quality of the hot mix asphalt is directly related to the quality of mixing in the annular chamber. Conventional mixing blades mounted on the drum are flat paddle-like elements mounted at approximately a 45.degree. angle to the horizontal. As the mixing blades pass through the hot mix asphalt laying on the bottom of the annular chamber, they cause the hot mix asphalt to move longitudinally toward the discharge outlet but do not lift the hot mix asphalt to any appreciable degree. By the time the mixing paddles travel to a position 90.degree. beyond their lowermost position, the hot mix has fallen off and the mixing paddles continue to travel around the annular chamber bare until they again contact the hot mix asphalt at the bottom of the annular chamber. Furthermore, at least some portion of the surface of the drum is continuously in contact with the hot mix asphalt in the bottom of the annular chamber and, in time, becomes coated with the mix, as well as with fines suspended in the annular chamber. Eventually, the build up of material on the outer surface of the rotating drum must be cleaned, requiring substantial down time.